Aramids are generally high temperature polymers. The commercial aramids, poly(m-phenylene isophthalamide) and poly(p-phenylene terephthalamide) are thermally stable polymers which on heating decompose before they melt. Hence, they are processed from solutions containing a minor proportion of polymer and, in the case of the meta aramid, large amounts of salt, such as CaCl.sub.2. They may be prepared by the acid chloride route which consists of reacting, for example, isophthaloyl chloride or terephthaloyl chloride, with an aromatic diamine, such as m-phenylene diamine in the presence of a solvent. In the manufacture of poly(m-phenylene isophthalamide), the HCl generated in the reaction is neutralized with a base such as Ca(OH).sub.2. The poly(m-phenylene isophthalamide) solution is then dry spun.
The acid chloride route is used for the synthesis of aramids because the normal melt condensation of aromatic diamines with aromatic dibasic acids does not occur or results in low molecular weight, infusible materials. The acid chloride route, on the other hand, has its own drawbacks such as (1) the chloride-related corrosion of equipment, and (2) the need to remove solvent and salts from fiber. Further, the polymers made by the acid-chloride route are usually non-meltable and thus are not melt-processible.
A worthwhile objective has been to prepare a salt-free aramid or aramid copolymer by a melt process. Such polymers and copolymers have the high glass transition temperature, T.sub.g, and good thermal stability of aramids and, at the same time, have the advantages of low cost melt-processibility to give products free of salt for superior electrical properties.
Because of their high thermal stability, and good electrical properties, aramid fibers are used to prepare variety of thermally resistant products, such as fire blocking fabrics and papers for electrical insulation in motors.